I found that some netizens are wrong about the ldr instruction in ARM.

ARM is a RISC structure. The movement of data from memory to CPU can only be completed through L/S instructions, that is, ldr/str instructions. For example, if you want to read data from somewhere in memory to a register, you can only use ldr, such as:

 

ldr r0, 0x12345678

 

That is, store the value at the address 0x12345678 into r0. However, mov cannot achieve this function. mov can only move data between registers, or move immediate values ​​into registers. This is the biggest difference from CISC architecture chips such as x86. There is no such instruction as ldr in x86, because the mov instruction of x86 can move data from memory to registers. There is


also the ldr pseudo-instruction. Although the ldr pseudo-instruction is very similar to the ARM ldr instruction, its function is slightly different. The ldr pseudo-instruction can add = before the immediate value to indicate that a value (usually an address) is written to a register, for example:

ldr r0, =0x12345678

 

 

In this way, the value 0x12345678 is written to r0. Therefore, the ldr pseudo-instruction is similar to mov. The only difference is that the mov instruction limits the length of the immediate number to 8 bits, which means it cannot exceed 512. The ldr pseudo-instruction does not have this limitation. If the ldr pseudo-instruction is used and the immediate number following it does not exceed 8 bits, then the ldr pseudo-instruction will be converted to a mov instruction during actual assembly. In fact, it is not accurate to say that the ldr instruction can load a 32-bit immediate number, because it is not actually this statement that loads a 32-bit immediate number. The real assembly code is to pass the value of a certain address to r1, which means that an address is needed to store the immediate number 0x12345678. And if this immediate number can be expressed in the form of a mov instruction, it will be actually replaced by mov by the compiler, for example:

ldr r1,=0x10

will become

mov r1,#0x10

In summary: The ldr pseudo-instruction is used to load a 32-bit immediate value or an address value into a specified register. When assembling a source program, the ldr pseudo-instruction is replaced by a suitable instruction by the compiler. If the loaded constant does not exceed the range of mov or mvn, the mov or mvn instruction is used to replace the ldr pseudo-instruction, otherwise the assembler puts the constant into a literal pool and uses a program-relative offset ldr instruction to read the constant from the literal pool. The ldr pseudo-instruction and the ldr instruction are not the same thing.